1. Field of the Invention
The present invention relates to a large-capacity storage medium and its recording and readout method.
2. Background Art
An increased amount of digital information has been handled year after year all over the world, resulting in a demand for digital storage devices with a larger recording capacity. Semiconductor memories such as flash memories, hard disk drives (HDDs), and optical recording media have been improved in their respective recording densities, resulting in lowering of the cost per bit. The future growth of the amount of digital information to be handled all over the world is expected to be explosive, thus requiring the development of storage devices that have still larger capacities.
The progress in microfabrication techniques allows the semiconductor memories to have smaller-sized memory cells of higher integration, so that both the recording density and the recording capacity of semiconductor memories are increasing. The size of each memory cell depends on the minimum size in fabrication. The fabrication technique of today can handle wires of 45-nm width each, but it is expected that wires of 32-nm width each and of approximately 22-nm width each will be handled in the future. In addition, not only the microfabrication techniques but also multilayer cells have been proposed as an idea of allowing the semiconductor memories to have larger capacities. An example of such proposal is disclosed in Japanese Patent Application Publication No. 2007-184084. The research and development thereof are progressing day by day.
The improvement of the recording density for HDDs has been achieved by making the size of each recording mark smaller. To record the smaller marks accurately, improvements have been achieved in the technique to localize a recording magnetic field produced by a recording head, as well as in the technique related to recording media to reduce variations in the mark shape. In addition, miniaturization of a readout magnetic field due to the smaller recording marks has been dealt with both by shortening the head-medium distance and by improving the sensitivity of the magnetic-field sensor. Moreover, as techniques to achieve a recording density of 1 Tb/in2 or higher, Applied Physics Letters, Vol. 74, pp. 2516-2518 discloses a technique known as “patterned media” in which patterns are formed on the medium in advance. Additionally, another technique proposed, for example, in IEEE Transaction on Magnetics, Vol. 42, pp. 2417-2421, is known as “heat-assisted magnetic recording” in which the medium is locally heated at the time of recording data thereon.
In the field of the optical recording, larger recording capacities have been accomplished by: the improvement in recording density, which is accomplished by using a light source of a shorter wavelength and by improving the numerical aperture of the objective lens; and the development of dual-layer recording media. Further development of multilayer media or of multiplexing techniques, and further improvement in recording density are expected to bring about the development of larger-capacity media. In addition, various ideas of the techniques, including the technique of further improving the recording density, the technique related to multilayer recording media, and the multiplexing technique such as the hologram, to give a CD-size disk of a 120-mm diameter a recording capacity of approximately 500 GB to 1 TB have been proposed, for example, in Optics Letters, Vol. 29, pp. 1402-1404. Particularly, the holographic recording, which is a method of accomplishing the larger-capacity data recording by means of the diffraction of light, is a promising method of accomplishing higher-speed data transfer because the recording data are acquired by using the diffraction images of plural pieces of data, that is, by using what is known as “parallel data processing.”    [Patent Document 1] Japanese Patent Application Publication No. 2007-184084    [Non-Patent Document 1] Applied Physics Letters, Vol. 74, pp. 2516-2518    [Non-Patent Document 2] IEEE Transaction on Magnetics, Vol. 42, pp. 2417-2421    [Non-Patent Document 3] Optics Letters, Vol. 29, pp. 1402-1404